MVC app runs in visual studio, fails on Server??

hi

Have run an app almost one month now on the iis server.  its MVC and so far so good.  today i added some minor changes to the code that add getting data from the database.   then a new var is cast of the model type, then 5 of the variables are filled from the data from the database call

public async Task<ActionResult> Create(int? id)
        {

the code works perfectly running in debug on the IDE/localhost.   but on the iis server?  it says only:

An error occurred while processing your request.

this only happens if I uncomment one or more of these lines referencing the variable with @ there in the View

<div class="form-group control-label col-md-3"><h3>  Customer Number:  @Model.CustNumber.ToString()</h3>  </div>

why does it not run on the server?  what kinds of diagnostic messages can be put in there?  how to find where its not runnng?  and i do not have access to the server log files

thanks for any thoughts or ideas